Machine dishwashing process

ABSTRACT

A machine dishwashing process for cleaning soiled crockery is provided. The process comprises: (1) optionally prewashing the crockery in a prewash zone; (2) washing the crockery in a wash zone with washing liquor; (3) spraying the crockery from the wash zone with a spraying solution; and (4) rinsing the crockery in a clear washing zone. The washing liquor includes used spraying solution from step (3). The spraying solution has an alkali content of at least 0.8 % by weight. An additive which is substantially free of alkali is supplied to the washing liquor to give a concentration in the washing liquor in the presence of the used spraying solution of at least 0.004 % by weight of complexing agent(s) and/or sequestering agent(s).

FIELD OF THE INVENTION

The invention relates to a machine dishwashing process for cleaningsoiled crockery and to the use of a composition as an additive in such aprocess.

BACKGROUND TO THE INVENTION

In a typical machine dishwashing process, crockery, such as tableware orcookware, soiled with food or other matter is subjected to a number oftreatments in a sequence of zones or cycles. In a main wash zone thesoiled crockery is usually sprayed with washing liquor at a pressuresufficient to detach much of the soil present on the crockery. The washzone may be preceded by a prewash zone in which the crockery is alsosprayed with water or with overflow from the wash zone. After the washzone the crockery is usually rinsed in one or more clear washing zonesusing fresh water.

Although a major factor in the effectiveness of the wash zone is themechanical action of the washing liquor on the soiled crockery, in mostcases the water pressure exerted is insufficient to detach completelyadherent soil such as food residues which have dried on, are firmlyadherent, or which contain colouring matter, especially when theycontain protein and/or starch. It is also found that a deposit can buildup on the crockery over the course of time during successive cleaningoperations. In order to make the cleaning of the crockery moreeffective, it is therefore usual to add into the process a cleaningsolution, which is usually supplied to the washing liquor as aconcentrated liquid cleaner.

The cleaning solution generally contains alkali, such as sodiumhydroxide or potassium hydroxide and is intended to remove the adherentsoil by swelling or partially dissolving it to facilitate rinsing. Thecleaning solution also contains complexing agents which are essentialfor sequestering metal ions present in hard water. Other additives mayinclude bleach to decolourise food residues which contain colouringmatter, surfactants to aid binding of dirt particles, and disinfectingcomponents.

EP-A-0282214 discloses an improvement in machine dishwashing processesin which the crockery is washed in the normal way in the main wash zoneand thereafter sprayed in a spraying step with a concentrated sprayingsolution before further rinsing. In the spraying step the crockery issubjected to a low volume/low intensity mist-like application ofconcentrated cleaning formulation and the crockery is allowed to remainin contact with the cleaning formulation for, say, at least ten secondsbefore rinsing. The concentrated cleaning formulation contains asessential ingredients both concentrated alkali and concentratedcomplexing agents.

A disadvantage of the arrangement of EP-A-0282214 is that the amounts ofraw materials such as caustic and complexing agents are not easy tocontrol. Typically, fresh water is applied in the rinse stage butconcentrated cleaning solutions are supplied during the spraying stepand in the main wash zone. In one typical arrangement, the water andother materials in the process pass in cascade-fashion in the rinse zoneto prewash zone direction, counter-current to the direction of thetransport of the crockery. Because concentrated raw materials aresupplied to various stages of the wash cycle it is very difficult,therefore, to measure and control simply and accurately the amount ofraw materials needed to be effective in cleaning the crockery. As aresult, to ensure effective cleaning there is a tendency to put in moreraw material than would actually be necessary. This adds to the runningcosts of the process and increases the amount of pollution when thewaste water is discharged into the environment. Moreover, because avariety of different water hardness conditions are encountered at thesite of use of the dishwashing process, it has hitherto been necessaryto offer to users a range of products. The user then has had to selectthe product with the best combination of water hardness and causticity.

SUMMARY OF THE INVENTION

The present invention provides a machine dishwashing process forcleaning soiled crockery, which comprises:

(1) optionally prewashing the crockery in a prewash zone;

(2) washing the crockery in a wash zone with washing liquor;

(3) spraying the crockery from the wash zone with a spraying solution;and

(4) rinsing the crockery in a clear washing zone; wherein the washingliquor includes used spraying solution from step (3), the sprayingsolution has an alkali content of at least 0.8% by weight, and anadditive which is substantially free of alkali is supplied to thewashing liquor to give a concentration in the washing liquor in thepresence of the used spraying solution of at least 0.004% by weight ofcomplexing agent(s) and/or sequestering agent(s).

In accordance with this process most or all of the alkali is supplied asspraying solution in the step of spraying the crockery. Contrary toprevious practice it has been found unnecessary to add additional alkalito the washing liquor. Instead, spraying solution used in the sprayingstep is effectively recycled to the washing liquor for the purpose ofwashing crockery in the main wash zone. This recycling is typicallyachieved using a wash tank or reservoir in known fashion from whichwashing liquor may be pumped onto the crockery.

Because the additive supplied to the washing liquor is substantiallyfree of alkali, the concentration of alkali in the washing liquor wouldbe less than 0.001% by weight if no used spraying solution were presentin the washing liquor, preferably less than 0.0004% by weight.

In a preferred embodiment, the spraying solution is substantially freeof complexing agents and sequestering agents. This separates the tasksof cleaning and of complex formation from one another and reduces theamount of alkalinity which is contained in the washing liquor, therebyreducing the cost of operating the process and the amount of pollutionin the waste water. In an alternative embodiment, the additive issupplied to the washing liquor with the spraying solution, preferably inan amount sufficient to prevent scale formation. In this embodiment, theadditive may be supplied to the washing liquor by dosing the sprayingsolution from a separate additive reservoir. In this way, the amounts ofspraying solution and additive can be separately controlled. Additivemay also be supplied directly to the washing liquor in the wash zone.

The high alkalinity of the cleaner used in the spraying zone ensurescomplete detachment of the soiling matter, especially starch, proteinand the additive facilitates complete detachment of colouringmatter-containing food residues on the crockery, which are thencompletely rinsed off in the actual washing zone. The additive is alsopresent to prevent the formation of scale inside the dishwashing machineand/or the spraying system.

By separating the tasks of cleaning and of complex formation from oneanother, operation of the process can occur in a more controlled manner.In particular, it is possible to deal on an individual basis withproblems of rinsing, scale formation and cleaning for different purposesby adjusting separately the amount of alkali and complexing and/orsequestering agents present in the process. The amount of alkali in theprocess may be supplied in accordance with the amount of water suppliedfor spraying. The amount of alkali in the process as a whole will dependonly on the rate of supply of alkali from the spraying solution in thespraying step. This enables a higher degree of control of the processthan has hitherto been achieved because the amount of alkali enteringthe process is regulated in direct response to the entering sprayingwater and substantially independent of the machine water consumption.

The higher the alkali content in the spraying solution, the moreeffective the cleaning of the soiled crockery. However, this has to bebalanced with the rinsability of the sprayed crockery and the cost ofthe alkali. Generally, the spraying solution has an alkali content of nomore than 1.5%. It is usual to supply the spraying solution as aconcentrated liquid cleaner which is diluted with water at a convenientpoint prior to spraying. The concentrated liquid cleaner typically hasan alkali content of at least 25% by weight and is diluted to aconcentration in the range 4% to 7% by weight for use in the process,advantageously to a concentration of about 5% by weight. Theconcentrated liquid cleaner generally has an alkali content of no morethan 50% by weight, normally in the range 30% to 50% by weight.

In the context of the present invention the term "alkali" encompassesalkalis typical in cleaning detergents and alkali builders such assilicates. Alkalis typical in cleaning detergents include sodiumhydroxide, potassium hydroxide, sodium carbonate and potassiumcarbonate. Sodium hydroxide and/or potassium hydroxide are preferredalkalis for use in the spraying solution.

The washing liquor comprises used spraying solution from the step ofspraying the crockery and the additive containing the complexingagent(s) and/or sequestering agent(s). For the avoidance of doubt, itshould be noted that there may be more than one complexing agent presentin the additive. Similarly, there may be more than one sequesteringagent.

Typically, the amount of complexing agent(s) and/or sequesteringagent(s) in the washing liquor is sufficient to prevent scale formationin the dishwashing machine and preferably does not exceed 0.05% byweight in the presence of the used spraying solution. Typically, theadditive is supplied to the washing liquor in concentrated form to givea dilution in the range 0.02% to 2% by weight, preferably in the range0.02% to 0.08% by weight. For example, where the washing liquor ispresent in a wash tank or reservoir, the additive may be supplied to thetank or reservoir in proportion to the water consumption of thedishwashing machine. Preferably, the additive is supplied to the washingliquor at a dilution in the range 0.03% to 0.05% by weight and mayadvantageously contain at least 50% by weight of the complexing agent(s)and/or sequestering agent(s). The above additive quantities applyparticularly to relatively soft water conditions. Where hard water isused, the quantities may need to be increased accordingly.

The complexing agent(s) and/or sequestering agent(s) may comprise achelator capable of sequestering metal ions and removing them fromactivity in solution by forming an inactive complex. Typical chelatorsinclude ethylenediamine tetra acetic acid (EDTA), nitrilotriacetic acid(NTA), tripolyphosphates, and their sodium and potassium salts.

The additive may additionally or alternatively comprise a component toinhibit calcium deposition, such as an acrylic or methacrylic polymerand/or phosphonic acids and/or their sodium and/or potassium salts. Bothof these polymers and phosphonates act to limit the formation ofcrystalline growth of calcium, thereby inhibiting its deposition. Thiscan be useful in preventing build up of limescale within the machinery.Typically, acrylic/methacrylic polymers are present in the additive atconcentrations of up to 5% by weight. Typically, phosphonic acids and/orphosphonates are also present in the additive at concentrations of up to5% by weight.

As a further possibility, the additive may comprise aphosphate/polyphosphate or a citrate. Whichever component(s) is selectedin the formulation of the additive, the alkali salt, neutral or acidform of the component may be used.

Each of the compositions supplied to the dishwashing machine may be inthe form of a liquid, slurry, powder or solid and the dosing system mustbe constructed accordingly to bring each composition into solution andinto the washtank/spraying system. Preferably, each composition will beformulated to have the highest possible amount of active ingredient init while still maintaining the most preferred physical form. The use offillers and other inessential ingredients is preferably avoided so as toprevent any negative environmental impact.

For example, the additive may be supplied as a powder having acomposition in the following range: 80 to 90% by weight NTA and/or EDTAand/or tripolyphosphates and/or their potassium or sodium salt; 4 to 9%by weight phosphonic acid and/or its sodium or potassium salt; and 4 to9% by weight acrylic or methacrylic polymer.

In the spraying step, cleaning solution is preferably applied to thecrockery as a fine spray. A fine, gentle mist-like spray is particularlydesirable. It is important that substantially the whole of the soiledsurface of the crockery is contacted by the sprayed cleaning solution. Acontact time of at least three seconds, preferably at least 8 seconds,is generally required for the sprayed cleaning solution to have thedesired chemical effect. The concentrated spraying solution must contactthe soiled crockery for a time sufficient to allow the soil to swell toenable it to be mechanically removed in the rinsing step. Generally acontact time of no more than 100 seconds is required, preferably ten totwelve seconds. Where the machine dishwasher is of the conveyor type,the speed of the conveyor belt may be chosen to given an appropriatecontact time before the crockery enters the rinse section. As a furtherfeature, the application of the cleaner solution may depend, forexample, on a light barrier influenced by crockery present on theconveyor belt.

The present invention further relates to the use of a compositioncomprising at least 50% by weight of a complexing agent and/orsequestering agent and less than 5% by weight of alkali as an additivein a machine dishwashing process. The composition may be used asdescribed herein to supply concentrated additive to the washing liquorand advantageously contains no more than 5% by weight of alkali,preferably less than 2% by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail by way of exampleonly and with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic diagram of a typical conveyor machinedishwasher which is usable in the present invention; and

FIG. 2A is a schematic diagram of a European warewashing apparatus.

FIG. 2B is a schematic diagram of an American type warewashingapparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a machine dishwasher of the conveyor type which operates bya cascade system. The dishwasher includes prewash zone 1, main wash zone2, power rinse zone 3 and rinse section 4. Conveyor 6 is used totransport crockery 7 through each of the zones and conventional sprayingunits 5 are provided to spray the crockery. The crockery is dried withdrying unit 8. Prewash tank 9, main wash tank 10 and power rinse tank 11are provided to supply to their respective spraying units solutionsappropriate to each zone.

In operation, soiled crockery is placed on the conveyor belt and isflooded in the prewash zone 1 with overflow water from the washing zonewhich is pumped to the spraying unit from prewash tank 9 and may have atemperature of about 40° C. Coarse food residues are thereby rinsed offthe crockery. The prewashed crockery enters main wash zone 2 and issprayed from the spraying unit with washing liquor from main wash tank10. The washing liquor will include used spraying solution from sprays12. Additive to the washing liquor is supplied to the main wash tankfrom a reservoir (not shown). Readily removable soil on the crockery isremoved in the main wash zone by a combination of the mechanical actionof the spraying and the chemical action of the recycled sprayingsolution.

The washed crockery now passes under the sprays 12 to remove anydifficult remaining soil. Sprays 12 impart a fine mist-like spray of theconcentrated spraying solution to the crockery and the speed of theconveyor is adjusted so that the sprayed crockery is not rinsed for atleast ten seconds. This enables the concentrated spraying solution tocause the remaining soil to swell and possibly dissolve under thechemical action of the spraying solution. The remaining swollen residuesare removed in the power rinse zone 3 by spraying with used rinse waterat a temperature of about 65° C. from the power rinse tank. The rinsewater originates from the final rinse section 4. After the power rinse,fresh water is supplied in spray jets 13 in final rinse section 4 at atemperature of about 85° C. The crockery is then subjected to dryingunder drying unit 8.

In accordance with this process, concentrated spraying solution wassupplied to sprays 12 at a concentration of 3% by weight and had analkali content of 30%. No complexing agents and/or sequestering agentswere present in the concentrated spraying solution.

Additive was supplied to the washing liquor at a concentration of 0.05%by weight containing 30% by weight EDTA, 30% NTA by weight, 3% by weightacrylic/methacrylic polymers, 2% by weight phosphonates, the balancebeing water. No free alkali was present in the additive.

Crockery cleaned according to this process was found to be free of soiland free of alkalinity.

Referring to FIG. 2, the present invention is equally applicable tomachine dishwashers operating by a washtank bypass system. According tothis system, the prewash zone is supplied with water from the powerrinse zone and not the wash zone. Fresh water is supplied to the washzone and allowed to drain. The present invention may be applied to thissystem in essentially the same way as it is, applied to the cascadesystem described above. Sprays for spraying the concentrated sprayingsolution are situated so that crockery already washed in the wash zoneis subsequently sprayed with the concentrated spraying solution.

What is claimed is:
 1. A machine dishwashing process for cleaning soiledcrockery, which comprises:(1) washing the crockery in a wash zone withwashing liquor; (2) spraying the crockery from the wash zone withwashing liquor; (3) rinsing the crockery in a clear washing zone;wherein the washing liquor includes used spraying solution from step(2), the spraying solution has an alkali content of at least 0.8% byweight, and an additive which is substantially free of alkali issupplied to the washing liquor to give a concentration in the washingliquor in the presence of the used spraying solution of at least 0.004%by weight of complexing agent(s) and sequestering agent(s).
 2. A processaccording to claim 1, wherein the spraying solution is substantiallyfree of complexing agents and/or sequestering agents.
 3. A processaccording to claim 1, wherein the spraying solution has an alkalicontent in the range 0.8% to 1.5% by weight.
 4. A process according toclaim 1, wherein the spraying solution is supplied as a concentratedliquid cleaner with an alkali content of at least 25% by weight anddiluted in the range 4% to 7% by weight.
 5. A process according to claim1, wherein the concentration of alkali in the washing liquor is lessthan 0.0004% by weight in the absence of the used spraying solution. 6.A process according to claim 1, wherein the amount of complexingagent(s) and/or sequestering agent(s) in the washing liquor does notexceed 0.05% by weight in the absence of the used spraying solution. 7.A process according to claim 1, wherein the additive is supplied to thewashing liquor at a dilution in the range 0.02% to 2% by weight.
 8. Aprocess according to claim 7, wherein the additive is supplied to thewashing liquor at a dilution in the range 0.03% to 0.05% by weight.
 9. Aprocess according to claim 8, wherein the additive contains an amount ofcomplexing agent(s) and/or sequestering agent(s) in the range 30% to 50%by weight.
 10. A process according to claim 1, wherein the additivefurther comprises a component to,inhibit calcium deposition.
 11. Aprocess according to claim 1, wherein the additive is supplied to thewashing liquor in step (2) with the spraying solution.
 12. A machinedishwashing process for cleaning soiled crockery, which comprises:(1)washing the crockery in a wash zone with washing liquor; (2) Sprayingthe crockery from the wash zone with a spraying solution; and (3)rinsing the crockery in a clear washing zone; wherein the washing liquorincludes used spraying solution from step (2) wherein the sprayingsolution has an alkali content of at least 0.8% by weight and issubstantially free of complexing agents and sequestering agents, andwherein an additive which is substantially free of alkali is supplied tothe washing liquor to give a concentration in the washing liquor in thepresence of the used spraying solution of at least 0.004% by weight ofcomplexing agent(s) and sequestering agent(s).
 13. A process accordingto claim 12, wherein the spraying solution has an alkali content in therange 0.8% to 1.5% by weight.
 14. A process according to claim 12,wherein the spraying solution is supplied as a concentrated liquidcleaner with an alkali content of at least 25% by weight and diluted inthe range 4% to 7% by weight.
 15. A process according to claim 12,wherein the concentration of alkali in the washing liquor is less than0.0004% by weight in the absence of the used spraying solution.
 16. Aprocess according to claim 12, wherein the amount of complexing agent(s)and sequestering agent(s) in the washing liquor does not exceed 0.05% byweight in the absence of the used spraying solution.
 17. A processaccording to claim 12, wherein the additive contains an amount ofcomplexing agent(s) and/or sequestering agent(s) in the range of 30% to50% by weight.
 18. A process according to claim 12, wherein the additiveis supplied to the washing liquor at a dilution in the range 0.02% to 2%by weight.
 19. A process according to claim 18, wherein the additive issupplied to the washing liquor at a dilution in the range 0.03% to 0.05%by weight.
 20. A process according to claim 19, wherein the additivecontains an amount of complexing agent(s) and/or sequestering agent(s)in the range 30% to 50% by weight.
 21. A process according to claim 12,wherein the additive further comprises a component to inhibit calciumdeposition.
 22. A process according to claim 12, wherein the additive issupplied to the washing liquor in step (2) with the spraying solution.23. A machine dishwashing process for cleaning soiled crockery, whichcomprises:(1) prewashing the crockery in a prewash zone; (2) washing thecrockery in a wash zone with washing liquor; (3) spraying the crockeryfrom the wash zone with a spraying solution; and (4) Rinsing thecrockery in a clear washing zone; wherein the washing liquor includesused spraying solution from step (3), the spraying solution has analkali content of at least 0.8% by weight, and an additive whichcomprises at least 50% by weight of a complexing agent and sequesteringagent and less than 5% by weight of alkali is supplied to the washingliquor to give a concentration in the washing liquor in the presence ofthe used spraying solution of at least 0.004% by weight of complexingagent(s) and sequestering agent(s).
 24. A process according to claim 23,wherein the additive comprises one or more complexing/sequesteringagents selected from EDTA, NTA, phosphonic acids, citric acid,phosphoric acid and polymers thereof, sodium and potassium saltsthereof, and acrylic/methacrylic polymers.
 25. A process according toclaim 23, wherein the additive comprises less than 2% by weight ofalkali.
 26. A process according to claim 23, wherein the additive is inthe form of a solid, powder, slurry or liquid.
 27. A process accordingto claim 21, wherein the additive comprises a powder comprising: 80 to90% by weight NTA and/or EDTA and/or a tripolyphosphate, and/or thepotassium or sodium salt thereof; 4 to 9% by weight phosphonic acidand/or its sodium or potassium salt; and 4 to 9% by weight acrylic ormethacrylic polymer.
 28. A machine dishwashing process for cleaningsoiled crockery, which comprises:(1) prewashing the crockery in aprewash zone; (2) washing the crockery in a wash zone with washingliquor; (3) spraying the crockery from the wash zone with a sprayingsolution; and (4) Rinsing the crockery in a clear washing zone; whereinthe washing liquor includes used spraying solution from step (3),wherein the spraying solution has an alkali content of at least 0.8% byweight and is substantially free of complexing agents and sequesteringagents, and wherein an additive which comprises at least 50% by weightof a complexing agent and sequestering agent and less than 5% by weightof alkali is supplied to the washing liquor to give a concentration inthe washing liquor in the presence of the used spraying solution of atleast 0.004% by weight of complexing agent(s) and sequestering agent(s).29. A process according to claim 28, wherein the additive comprises oneor more complexing/sequestering agents selected from EDTA, NTA,phosphonic acids, citric acid, phosphoric acid and polymers thereof,sodium and potassium salts thereof, and acrylic/methacrylic polymers.30. A process according to claim 28, wherein the additive comprises lessthan 2% by weight of alkali.
 31. A process according to claim 28,wherein the additive is in the form of a solid, powder, slurry orliquid.
 32. A process according to claim 26, wherein the additivecomprises a powder comprising: 80 to 90% by weight NTA and/or EDTAand/or a tripolyphosphate, and/or the potassium or sodium salt thereof;4 to 9% by weight phosphonic acid and/or its sodium or potassium salt;and 4 to 9% by weight acrylic or methacrylic polymer.